This invention concerns a method for heat treating rolled stock, and the device to achieve the method.
To be more exact, the invention is applied to rolled products leaving the rolling step and before they are collected and/or wound into compact structures such as coils, rolls, bundles or packs.
The rolled stock to which the invention is applied may belong either to the class of materials which require a process of surface hardening followed by tempering, and also that class of materials wherein it is not desired to obtain surface structures which are typical of a hardening process and on which cooling is performed with speeds lower than the speed at which the original austenitic structure is transformed into a martensitic structure.
The state of the art covers the problems relating to cooling treatments carried out on rolled stock leaving the rolling train, which also have the function of guaranteeing that the product has optimum characteristics of quality and structure, both surface and internal.
In the state of the art we can identify two classes of materials which are of interest.
The first is rolled stock which, as it leaves the last rolling pass with an austenitic crystalline structure, is subjected to surface hardening and subsequent tempering, with the crystalline structure being transformed into a martensitic, or at most bainitic, surface structure in the surface and sub-surface layers.
The second is rolled stock which, as it leaves the last rolling pass, is cooled with different criteria but in any case with the purpose of not obtaining structures which are typical of hardening and for which the transformation of the austenitic structure in the relative stable structure is begun and completed after the stock has been discharged from the rolling line, typically in a cooling bed or plane.
For the first class of products, it is well-known that a rapid cooling may be applied to the product when it leaves the last rolling pass so as to exceed a determined cooling speed, or critical speed, above which crystalline micro-structures are formed, characterised by great hardness and resistance.
This rapid cooling, which hardens the surface of the product, obtains a surface area wherein there are very fine martensitic structures which are typical of the hardening process. The martensitic structures are obtained by suppressing the transformation of austenite because of the rapidity of the fall in temperature.
At the same time, bainitic structures are obtained below the surface of the rolled stock, while in the core of the product, where the removal of heat is slower and the temperature is maintained higher, pearlitic structures are obtained which are less resistant but are extremely tough.
The cooling may be regulated so as to obtain different depths of treatment and thus, by balancing the mechanical properties of the different structures which are created at the different depths of the product, to achieve the best balance of resistance and toughness of the finished product. With these opportunities of regulating the treatment, in terms of at least duration and intensity of cooling, it is possible to process materials with different diameters and different chemical compositions in order to obtain the same mechanical and quality requirements on different types of products.
By using the heat treatment as described above, it is possible to obtain the minimum mechanical characteristics as established by different national legislations without requiring the use of binding elements which would inevitably increase the cost of the product. Moreover, given their limited carbon content, the welding characteristics of the rolled stock are maintained.
Therefore, the compromise between mechanical resistance and toughness of the product, so as to satisfy the required standards of quality, is substantially based on the parameters of duration and intensity of cooling applied.
These parameters not only define the specific penetration of the hardening process, they also determine the level of heat which is established on the rolled product when the heat of the core spreads towards the surface areas and equalises the temperature over the whole section of the rolled stock.
It is extremely important to know and define the level of heat since it measures the efficiency of the tempering of the martensitic structures obtained in the surface areas of the product. The tempering takes place during at least part of the temperature-equalisation step which follows the rapid cooling.
At the end of the temperature-equalisation step, in the subsequent air cooling step, for example carried out in the cooling bed, wherein the temperature at all points of the product begins to fall, the tempering process continues; at this stage of the process, the hardness of the surface areas is redimensioned, and at the same time there is a considerable increase in the toughness.
In those cases when the cooling is carried out in the cooling bed, the speed at which the temperature falls is in any case sufficiently high to limit the negative effects of an excessive tempering on the mechanical characteristics of the surface of the product. If on the contrary the product is immediately arranged into compact structures, for example, wound into rolls or coils, the reduction of surface exposed to heat exchange through convection or radiance causes a considerable slow-down in the cooling treatment with a consequent increase in times.
This increase in the cooling times causes a greater efficiency and influence of the hardening process and therefore a deterioration in the mechanical characteristics of the material which is often excessive and unacceptable.
For this reason, in the state of the art the rolled product is always subjected to a step of natural, air cooling, and it is only when this cooling is completed in times compatible with balancing the consequences of the tempering process, and the crystalline structure is stabilised, that the product is collected and discharged.
This involves an obvious and enormous increase in the space needed on the line.
For the second class of materials, which consists of products which are not subject to a hardening process, it is well-known in the state of the art to cool the product downstream of the last rolling pass to different degrees but in any case in a manner such as to exclude the formation of those structures which are typically produced by hardening, such as martensite or bainite.
In these cases, the cooling speed is therefore less than the speed which leads to martensitic transformation and the heat is removed from the rolled stock in such a manner so as not to create palpable differences between the surface area and the core of the product.
The transformation of the austenite in the stable crystalline structures is therefore generally achieved with mechanisms of nucleation and growth which typically need relatively long times.
The phases in the finished product, along its whole section, will be ferrite and pearlite in percentages which will depend on the chemical composition of the raw material. In some cases, for steel alloys, there may also be bainite.
In this case, it is above all a uniformity of structure which is desired, while the level of the mechanical properties required may differ considerably due to the different types of steel treated.
According to the properties required the cooling process may be carried out in different ways; however, in all cases, as it is transported on the line, the rolled stock is given the time necessary for it to cool naturally in air so that the phase transformations of the austenite can take place in the stable structures.
On the contrary, in the event that the product is collected in compact structures, such as rolls and coils, immediately after the rolling process, the reduction of the surface exposed to heat convection and radiance causes a considerable slow-down in the cooling process.
This modifies in a substantial manner the heat cycle of the rolled stock and leads to modifications of the micro-structure which inevitably affect the final properties thereof.
To be more exact, there may be modifications to the following: the relative quantities of the phases present, the micro-structure thereof and the size of the crystalline grains. These modifications can be such as to render the technological qualities of the rolled stock unacceptable.
Therefore, for both classes of materials mentioned, both those products which have to be subjected to hardening and tempering, and also those where it is not desired to obtain structures typical of hardening, there is a common technical problem which hinders the immediate collection of the rolled stock into compact structures and, on the contrary, in order to obtain the required characteristics of quality and technology, requires a long section of natural air cooling to be performed.
The present applicants have designed, tested and embodied this invention to overcome the shortcomings of the state of the art, and to obtain further advantages.